The present invention regards a switching circuit having a plurality of sets of switchable transceiver terminals for connecting individually to a common antenna, each set comprising a plurality of PIN diodes and a DC network for applying switching bias voltage from a DC source for connecting a selected one of said sets to said common antenna and for disconnecting non-selected sets from said antenna.
The invention is in particular directed to problems appearing in mobile telephones using two different frequency bands, for example the EGSM 900 system and the GSM 1800 system. The invention is, however, not limited to this use or even to the use in cellular telephony.
A switching circuit is known from Japanese no. 01-029130, where two transmitters having a common antenna are mutually isolated by means of PIN diodes. On the other hand, there is no question of transceivers in this publication, the receiver function not being covered.
It is an object of the present invention to obtain a switching circuit of the kind recited above, and where reverse bias can be arranged with few components and without the necessity of an auxilliary polarity source.
It is another object to prevent harmonic generation in inactive branches of a multi-band PIN diode antenna switch. This problem of harmonic generation is particularly acute in the case where one frequency band has twice the frequency of the other, such that e.g. the first overtone of the lower frequency falls within the band of the higher frequency. Thus, even if the transmitters are connected to the antenna via the respective terminals of a duplex filter, a sufficient proportion of the transmitter signal may reach the other(s), where the respective fast PIN diodes if unbiased will be able to create spurious overtones which are sent back and may reach the antenna.
A forward biasing in both branches of a two-band system, thus avoiding quiescent PIN diodes creating spurious signals, might work if there are two different transmitter power amplifiers and only one of them is activated at the same time. For mobile units, where size and weight are kept low, it is sometimes advantageous to have only one power amplifier having two outputs and arranged for feeding transmitter energy in either frequency band. Then, the power amplifier as working on the lower frequency will still output some harmonics or spurious signals at awkward frequencies. It is therefore another object of the invention to obtain clean transmitting signals without harmonic distorsions.
The said objects are satisfied, according to the present invention, by arranging in a switching circuit as recited such that at least two PIN diodes belonging to the same set are connected in series and similarly directed, that the DC network therof is connected for enabling a forward current through said PIN diodes of the set, that all said DC networks have a common DC terminal which is connectable via a bias resistor to a terminal of a DC source having the opposite polarity relative to said switching bias voltage, and whereby activating by a forward bias current in one of said DC networks will create a bias voltage in said bias resistor when connected, for reverse biasing the PIN diodes of a set not so activated.
Although any number of transceiver devices may be arranged switchable, preferable through multiplex filters, to a common antenna, the invention is particularly useful in a cellular mobile telephone arranged for communication in alternative frequency bands, like the EGSM 900 (880-915 and 925-960 Mhz for transmitting and receiving respectively) and the GSM 1800 (1710-1785 and 1805-1880 Mhz for transmitting and receiving respectively). Similar problems exist in other communications systems.
In an advantageous embodiment, the high-frequency components, including a diplex antenna filter, of the switching circuit are arranged in a sole modular component, which may be mounted on a printed circuit board, the components for DC biasing arranged around on that board. The modular component can then be made to exacting tolerances as demanded in microwave equipment.
By setting adequate bias voltages, it is then possible to connect each transmitter to the antenna under disconnecting the other(s), and to disconnect all transmitters while connecting the respective receiver terminals to the antenna. Thus, in the case of a double transceiver band embodiment, the bias setting may have at least three different states, namely, a first state for enabling a first set for transmitting in a first frequency band and deactivating a second set by reverse biasing its PIN diodes, a second state for enabling the second set for transmitting in a second frequency band and deactivating the first set by reverse biasing its PIN diodes, and a third state where the PIN diodes in both sets are reverse biased, and where receiver terminals are connected to the respective terminals of the duplex filter.